The present invention relates to a plasma processing method, particularly, to plasma processing method in which a reactive gas is introduced into a process chamber for generating a plasma within the process chamber so as to decompose the reactive gas and to process a target substrate.
In a plasma processing apparatus, a target substrate is processed in general by the procedures described below. In the first step, a reactive process gas is introduced into a process chamber exhausted into a vacuum state, followed by applying a high frequency to the reactive process gas so as to generate a plasma and decompose and excite the gas. The target substrate is processed by using the active species thus formed, which has a high reactivity. In a plasma processing apparatus such as a plasma etching apparatus or a plasma CVD apparatus, all the process gas introduced into the process chamber is not consumed for the reaction with the substrate. It is more reasonable to state that a major portion of the process gas introduced into the process chamber is not used for the reaction with the target substrate so as to be exhausted to the outside by an exhaust apparatus. Under the circumstances, it is of high importance to improve the rate of utilization of the process gas so as to decrease the cost of the process gas occupied in the manufacturing cost in the etching or CVD step.
In the etching and CVD processes, various kinds of PFC gases each having a high GWP (global warming potential) are used in large amounts as the process gas and as the cleaning gas of the apparatus. For suppressing the warming of the earth, it is of high importance for the manufacturers of the semiconductor device to take measures for suppressing the discharged amount of the PFC gases. However, it is considered very difficult nowadays to find a substitute gas having a low GWP, having a high safety, and performing the function at least equivalent to that of the PFC gas. It should also be noted that many of the PFC gases are very stable chemically, requiring a novel technology and an apparatus for decomposing and removing the unreacted PFC gas which is contained in the exhaust gas discharged from the process chamber. In other words, a new investment is required for decomposing and removing the unreacted PFC gas.
In the manufacturing process of a semiconductor device, a particularly large amount of the PFC gas is used in the step of etching a silicon oxide film. However, the amount of the PFC gas used for a single apparatus is small, i.e., about 10 to several hundred cubic centimeters (cc) in each processing. In addition, the PFC gas supplied to the process chamber is partly consumed for the reaction with the target substrate, with the result that the amount of the unreacted PFC gas, which is discharged to the outside and should be decomposed and removed, is further decreased. It follows that it is economically impossible to arrange an apparatus for decomposing and removing the unreacted PFC gas for every etching apparatus. Naturally, it is highly important to increase the utilization efficiency of the process gas so as to decrease the discharge amount of the unreacted PFC gas.
An apparatus effective for overcoming the difficulty is proposed in, for example, Japanese Patent Disclosure (Kokai) No. 9-251981. This prior art is directed to a plasma etching apparatus and a plasma CVD apparatus in which a substrate is processed within a vacuum process chamber. It is proposed to arrange a circulating pipe connecting the pipe on the discharge side to the process chamber so as to return a part of the discharged gas back into the process chamber for reuse.
A silicon wafer is etched in a DRM type plasma etching apparatus equipped with a circulating mechanism while circulating a process gas. FIG. 1 schematically shows the construction of the etching apparatus used. As shown in the drawing, a parallel plate type plasma generating apparatus comprising a cathode electrode 102 and an anode electrode 103, which are arranged to face each other, is arranged within a process chamber 101, and a parallel magnetic field is generated within the process chamber 1 by using a magnetic field applying apparatus (not shown). A target substrate 104, which is to be processed, is disposed on the cathode electrode 102, and a high frequency power source 106 is connected to the cathode electrode 102 with a matching circuit 105 interposed therebetween. On the other hand, a shower nozzle 107 for uniformly supplying a process gas onto the target substrate 104 is incorporated in the anode electrode 103. A gas cylinder 109 used as a supply source of the process gas is connected to the shower nozzle 107 with at least one flow rate controller 108 interposed therebetween. The apparatus shown in the drawing includes one flow rate controller 108 and one gas cylinder 109. However, it is possible to determine appropriately the number of flow rate controllers and the number of steel gas cylinders, as required.
A turbo molecular pump 111 is connected to the process chamber 101 with a pressure control valve 110 interposed therebetween, and a dry pump 112 is connected to the exhaust side of the turbo molecular pump 111. Also, a circulating pipe 113 is arranged between the exhaust side of the turbo molecular pump 111 and the process chamber 101. A flow rate control valve 114 is mounted to the circulating pipe 113 for controlling the circulating rate. Also, another flow rate control valve 115 is arranged upstream of the dry pump 112.
For processing the target substrate by using the apparatus shown in the drawing, gases of C4F8, CO, Ar and CO2 are supplied at a predetermined flow rate ratio from the gas cylinder 109 into the process chamber 101 through the flow rate controller 108. At the same time, the flow rate control valve 114 mounted to the circulating pipe 113 is opened, and the degree of opening of the flow rate control valve 115 arranged upstream of the dry pump 112 is decreased. A part of the exhaust gas discharged from the process chamber 101 by the operation of the turbo molecular pump 111 is returned into the process chamber through the circulating pipe 113. In other words, since the exhausted process gas is utilized again so as to decrease the amount of the newly introduced gas, compared with the case where the exhaust gas is not returned partly into the process chamber.
The apparatus shown in FIG. 1 permits increasing the utilization rate of the process gas because the process gas is partly circulated within the process chamber so as to decrease the amount of the process gas used. However, the etching characteristics are changed and deteriorated by the circulation of the process gas. FIG. 2 is a graph showing the relationship between the silicon dioxide etching rate and the process gas circulation rate.
Specifically, FIG. 2 shows the change in the etching characteristics in the SAC (Self-Aligned Contact) forming process of an oxide film, covering the case where the circulating rate of the process gas is changed from 0% to 80%. The circulating rate was changed by controlling the valve 115 while maintaining constant the total gas flow rate, i.e., the sum of the newly introduced gas flow rate and the circulated gas flow rate, into the process chamber. On the other hand, the change in the etching characteristics was evaluated in terms of the SiO2 etching rate and the selectivity to resist.
As apparent from the graph of FIG. 2, each of the etching rate and the selectivity was lowered with increase in the circulating rate. It should be noted in this connection that the circulating gas introduced again into the process chamber 101 and the gas newly introduced into the process chamber 101 differ from each other in the composition and the flow rate ratio.
FIG. 3 shows the result of the analysis of the components of the exhaust gas, covering the case where the etching was performed with the circulating ratio set at 0%. In FIG. 3, CO and Ar having a relatively high flow rate are omitted. The introduced gas, i.e., the base gas C4F8, which is a PFC gas, is partly decomposed by the plasma discharge so as to be involved in the etching reaction and to be adsorbed on the inner wall of the process chamber and partly remains unreacted so as to be discharged as it is to the outside as the exhaust gas. On the other hand, FIG. 3 clearly shows that the reaction products formed by the discharge and the etching reaction are also exhausted to the outside.
In order to avoid the problem in terms of the change and deterioration of the processing characteristics, proposed is a method of mounting a gas refining mechanism to the circulating pipe such that the exhaust gas is refined and, then, returned into the process chamber. For example, proposed in each of Japanese Patent Publication (Kokoku) No. 7-36886 and Japanese Patent No. 2854240 is an idea of mounting to the circulating pipe a refining mechanism such as an adsorption tower, a dehydrating tower, a decarboxylation tower, and a filter and a gas analyzer for monitoring the state of the refining mechanism so as to regenerate a gas exactly equal to the newly introduced gas in the composition and the component ratio. It is proposed that the regenerated gas is introduced into the process chamber so as to maintain the characteristics of the process gas. Another idea is proposed in, for example, Japanese Patent Publication No. 5-40031. Specifically, it is proposed to refine the exhaust gas to obtain another raw material that should originally be stored in another gas cylinder in place of refining the exhaust gas to obtain a refined gas exactly equal in composition to the newly introduced gas as in the method of forming a silicon film. Each of these prior arts is intended to maintain the processing characteristics on the basis that the gas returned into the process chamber is known. In order to achieve the object, required are many costly facilities in addition to the gas circulating mechanism. Since in the plasma etching apparatus, a small amount of a high purity gas is supplied into the process chamber while controlling the gas amount in units of sccm or less, the technology proposed in the prior arts pointed out above is not practical.
Under the circumstances, it is practical and economical in the plasma etching process to return the exhaust gas as it is into the process chamber for reuse without particularly refining the components of the exhaust gas. In this case, however, a gas having an unknown gas component mixed therein at an unknown mixing ratio is introduced into the process chamber, making it necessary to set the gas flow rate condition appropriately.
Where the conditions of an etching process are newly established on the basis that a gas is circulated at a fixed circulating rate, a problem particularly different from that of the prior art does not take place. However, the work to reconstruct the flow rate conditions of the gas introduction starting with zero is generated separately. The particular work is required in the case where the process, in which measures are not taken for the environment relating to the gas circulation, is changed into another process in which a gas is circulated as a measure for the environment. In other words, the particular work is required in the case where the etching characteristics under the process conditions constructed without circulating the gas are to be reproduced while circulating the gas.
To be more specific, a target substrate is etched by changing in a matrix fashion the flow rate of each of the introduced gases including C4F8, O2, CO and Ar. Then, it is necessary to determine the new gas introducing conditions while confirming the etching characteristics such as the etching rate, the selectivity and the shape of the etched target substrate by measuring the film thickness scores of times and by observing the cross sections with an SEM 10 to 20 times. It is possible to decrease markedly the discharge amount of the PFC gas by these operations. However, a tremendously long time and high cost are required for finding the new gas introducing conditions. In other words, required is a tremendously high material cost including not only the wafer cost but also the gas cost.
In addition, if the circulation ratio is increased in order to improve the effect of suppressing the PFC gas discharge, the ratio of the circulated gas to the newly introduced gas is increased, with the result that the influence given by the circulated gas component to the process is relatively increased. Under the circulation of, for example, 80%, the change in the flow rate of the newly introduced gas by 1 sccm corresponds to the change in the flow rate of the circulated gas by 4 sccm. It follows that it is necessary to control highly accurately the flow rate of the introduced gas and to maintain the controlled flow rate.